Systems which coherently sum a series of tapped delay lines are relevant to a wide range of signal processing applications, transverse filtering being a prominent example. Another example is that of decoy repeaters. An object will modify any signal reflected from it according to the object""s shape, and the object""s velocity relative to the signal. This permits a hostile interrogator to identify the nature of such objects, which, if the objects are military platforms such as warships or aircraft, is not desirable. One solution has been to artificially synthesize fake characteristic echo signatures in response to receipt of an interrogating signal. Thus, for example, a series of decoy buoys deployed at sea could simulate the presence of a naval flotilla, and thereby potentially disrupt enemy plans.
FIG. 1 illustrates broadly how this is done for a ship 5 and an aircraft 3, in the line of sight of an interrogating signal 2. For illustrative purposes, signal 2 can be a radar pulse, but could as well be any linear signal pulse, of which sonar or acoustic signals are other examples. As signal 2 hits aircraft 3 and ship 5, it bounces off of their major reflective surfaces, which, for ship 5 are the hull 4, superstructure 6, and smokestack 8, and for the aircraft are the nose 9 and the wings 7. The echo from craft 3, 5 will be the superposition of the echoes from surfaces 4, 6, 8, and 9, 7, and because these surfaces are at different places along the line of sight of signal 2, the superimposed reflections will be out of phase with one another by the differing times of flight of signal 2 to each reflecting surface. This tends to lengthen the echo by an amount equal to the round trip time of flight of signal 2 between the nearest and farthest major reflector, in the case of ship 5 hull 4 and smoke stack 8, and to make the echo of varying magnitude as dictated by the varying radar cross sections of the reflecting surfaces. Furthermore, movement of aircraft 3 or ship 5 relative to signal 2 will Doppler shift the returned echos. Thus any platform which reflects signal will in effect frequency modulate signal 2, such that the returned echoes permit an interrogator to infer the nature and motion of the platform. The most common way to detect a Doppler shift is, responsive to a series of interrogation pulses, compare echoes from consecutive pulses. Thus, an imaging interrogator, such as a search radar, SAR, ISAR, etc., would infer Doppler by comparing consecutive echoes, and do so on a range bin by range bin basis. The interrogator can infer strength of reflection, e.g. radar cross-section, in a range bin from only one echo returned from the range bin by checking echo strength, although as a matter of prudence, it would likely check several such echoes to ensure that echo strength doesn""t vary greatly.
Any credible repeater decoy must simulate the temporal lengthening and amplitude modulation caused by plural, recessed, reflective surfaces, and a simulate a realistic Doppler shift for each surface.
Conventionally this is done by analog systems which receive an interrogating signal and pass it through a length of cable having serial taps along its length, one tap per range bin (also called range cell, or downrange range cell). Each tap modulates the signal in amplitude and/or frequency to simulate reflection from the reflective surfaces within that range bin. Total path length of signals traversing the respective taps are selected to correspond to the differing times of flight of the interrogating signal to the respective range bins. Finally, the signals from the taps are summed, and the signal thus synthesized is retransmitted. In this manner, the system returns what appears to be an echo from an object located within the selected range bins, and having a signature indicative of the object to be simulated, e.g. a ship or aircraft in motion.
Unfortunately, analog systems have drawbacks which limit their usefulness as decoys. They are inherently noisy, and can hold an incoming signal only a short time for processing before the signal deteriorates below noise. This limits system bandwidth, and permits effective simulation of only small objects. Further, analog systems are costly and very bulky, the latter being a particular concern for military platforms, where space is extremely limited. Finally, analog systems cannot readily change operating parameters such as relative delays among taps, or the amount of modulation in the various taps. This means that analog repeaters cannot switch among different simulated objects on the fly, but rather must typically be fabricated for one specific target.
Accordingly, an object of the invention is to increase the bandwidth of tapped delay line processors of the kind above described.
Another object is to permit such delay line processors to hold received signals as long as necessary for a given application.
Another object is to increase the size of objects which such delay lines can simulate when used as repeater decoys.
Another object is to reduce the size and cost of such delay line processors.
In accordance with these and other objects made apparent hereinafter, the invention concerns a signal synthesizer which has a digital radio frequency memory (DRFM), and an associated digital processing circuit having a plurality of tapped delay lines, a summer disposed to sum the output of the delay lines, and signal modulator in each of the delay lines. A DRFM is a semiconductor device which can rapidly and permanently record digital information, most notably digitized samples of an incoming signal, and read it back equally rapidly when needed, as well as carry other circuitry. Because of this, the processing circuitry associated with the DRFM can be digital also, with its attendant values of speed, and hence greater bandwidth, reliability, small size, and modest cost. Because the DRFM can hold data indefinitely, the duration of the synthesized signal is not limited, as with analog systems, thus permitting (in the example of FIG. 1) simulation of larger objects by adding more taps to accommodate more range bins. Because the associated circuitry can be digital, and most especially because the circuitry can be dedicated to its processing task, rather than requiring extensive programming to perform its tasks, the speed of the synthesizer can be especially great. In a preferred embodiment, the associated circuitry is made part of the DRFM on the same monolithic chip in order to increase synthesizer speed even more. This is in contrast to a computer, or programmable processor, which, in conjunction with a fast and permanent memory like a DRFM, could in principle do the necessary processing. But the time needed to execute the large number of programming instructions necessary to process data makes this far less desirable than the invention, and, for the specific problem of decoy repeaters, largely ineffective.
Please note that, although the name DRFM suggests radio frequency signals, and thus suggests use in RF, radar, and other microwave applications, use of the term DRFM is not intended to limit the invention to these applications. Rather, the term DRFM has come to be associated with a specific class of device in the semiconductor art, and is here used only to denominate such devices, not suggest limitations on their uses.
These and other objects are further understood from the following detailed description of particular embodiments of the invention. It is understood, however, that the invention is capable of extended application beyond the precise details of these embodiments. Changes and modifications can be made to the embodiments that do not affect the spirit of the invention, nor exceed its scope, as expressed in the appended claims. The embodiments are described with particular reference to the accompanying drawings, wherein: